Systems for monitoring the condition of materials (i.e., material level, pressure, proximity, etc.) at a plurality of discrete condition points are known. These systems generally fall into two categories.
Systems of the first category generally operate by converting a measurable variable, such as capacitance or admittance, which is proportional to a material condition being studied, such as height, into an analog signal proportional to the measured variable. The analog signal is then compared to a plurality of discrete signals or converted to a digital signal which is compared with the plurality of discrete values. In one multi-point monitoring system of the first category known to the inventors, radio frequency admittance measurements are converted to an analog signal which is routed to a plurality of current-sensing set point relays. A major problem with such systems is that each stage of operation upon the measured variable, such as amplification, conversion, or comparison, has the potential of introducing spurious effects. In combination, these effects may be sufficient to generate a false output at the comparison stage. It would be necessary to use relatively expensive, very high quality circuit components with very narrow operating tolerances to prevent the introduction of spurious effects.
Systems of the second type incorporate a bridge which compares the measured variable corresponding to the material condition being studied with a reference variable. The output of the bridge is thereafter processed to provide a useful signal. The major advantage of this type of system over the first is that the introduction of spurious effects from the processing circuitry has limited impact because the critical step, bridge comparison, occurs before amplification and subsequent signal processing steps. Known multiple set point monitoring systems which incorporate a bridge are typically operated at balance at a single point and off balance for other points. The balance point may or may not be a set point of interest.
A problem in all known multiple set point systems of both categories is that potentially interfering signals may be present either where the material is being measured or where the resulting measurement signal is processed. It may be necessary to guard the measurement signal against these interfering signals to prevent a false reading of the system. It is considerably more difficult to guard a signal which takes on a wide range of values than it is to guard a signal which is only of interest at a single value. This particular difference is so significant that it allows relatively inexpensive single-point monitoring systems to outperform more expensive, analog monitoring systems in which interfering signals are present. Such signals might arise, for example, by residual coatings of conductive materials on the sensing element in radio frequency admittance-type material level systems.
Single set point bridge systems are desirably balanced near the set point being examined. In single set point guarded systems, the reference potential of the bridge typically provides a low impedance source of voltage which will be the same as the sensing element voltage when the bridge is balanced. This allows the reference potential to be effectively used to provide a guard voltage to a guard electrode shielding the sensing element. Unfortunately, when the bridge is not balanced, the voltage at the sensing electrodes departs from the reference potential. This renders a guard electrode coupled with the reference potential less effective.
A number of other single set point, material condition monitoring systems are known which are capable of generating a plurality of sequential reference signals for calibration. For example, commonly assigned U.S. Pat. No. 4,485,673 to Stern discloses a single set point two-wire level measuring system utilizing a pair of admittance sensitive sensors, each including a capacitive balance bridge. A pair of set point calibration systems is provided, each with a multiplicity of capacitances. The capacitances are selectively coupled to each of the two bridge networks provided for set point calibration. The system is thereafter operated with fixed reference capacitances. U.S. Pat. No. 4,555,941 describes a material level detection system incorporating an automatic calibration circuit in which capacitances are automatically switched into an LC or resonant circuit. Both systems are unsuited for multi-set point operation in their present configuration. Each lacks means for changing the reference set point during monitoring operations. Each also lacks a feedback loop which would adapt to changes in the reference set point during monitoring operations.
U.S. Pat. No. 4,063,447 to Mathison discloses a monitoring system having a bridge network in which a reference current source is adjusted automatically to compensate for system drift. It is also unsuited for multi-set point operation in its given configuration.
U.S. Pat. No. 4,383,444 discloses a capacitance level detection system of the first category in which a measurement capacitance is processed and then compared to a reference capacitance. A microprocessor is provided as part of the system for automatic calibration. There is no teaching or suggestion that this system is useful for or capable of multi-set point operation. Being a first category type system, it is also susceptible to spurious effects during measurement and signal processing.
Lastly, adjustable differential set point monitoring systems are known. Such systems have typically been provided for monitoring sumps and the like. A bridge is provided with a material condition responsive sensor for developing a variable, material condition dependent admittance. A primary capacitor provides a reference admittance. When the bridge output switches as a result of low material related admittance from the sensor, which occurs when the materials in the sump falls below a first level, a second capacitor is coupled with the first capacitor to raise the reference admittance. The bridge output will not switch until the material has risen to a second level above the first. The single output signal of such systems is indefinite as to a particular set point as it may be related to either of two reference capacitances.